diff --git a/src/branch2.hpp b/src/branch2.hpp
index 889a14e7..643d7af4 100644
--- a/src/branch2.hpp
+++ b/src/branch2.hpp
@@ -19,7 +19,7 @@ public:
   Branch2(Branch2 const &);
   Branch2(Branch2 &&);
   Branch2(toml::value const &value,
-          uint64_t const     min_free_space);
+          uint64_t const     min_free_space = 0);
   ~Branch2();
   Branch2& operator=(Branch2 const &);